Wiring Woes

The black appearance is oxidation of the copper from moisture or overload, probably the later as other circuits use this as a common earth.  It should really be cut back to clean copper but that could be the whole remaining length. If it is not replaced, when it fails a number of circuits will stop so will be a major problem if on the road at the time.

Quick fix is to abandon that core and run a thicker additional wire under your caravan to your new junction boxes.

If the wire for T13 (white/red) is discoloured, but not the wire for T9 (orange) then there is a possibility of 'economical' wiring of the caravan car, and T13 is doing more than its fair share. T13 and T9 should only be the permanent 12 supply to the caravan and would (should) do nothing while you are towing. T11 (white/black) with T10 should be working to supply your fridge and battery charging, they will work while you are towing, and do nothing on site. If anything is going to overload, it would be the T11 and T10 pair.

Also just do a quick check on the car, T3 (white), T11 and T13 should all be wired independently back to a good chassis point in the car.

There is a chance that T13 is doing the work of T3 and T11, which will 'work' (for some alternative definition of work).

Also, check the replacement cable that you fit to the 13 pin, I have seen cables made up with all the came cable rating, (popular on an online site that has a relationship with rain forests). Proper cable will have different thicknesses for the different current loading.

The permanent 12v supply should provide the battery charging current while towing. It will normally be connected in the caravan pdu by a relay operated by the fridge supply when car engine is running.

If the fridge and battery are connected to the same supply cable (t10) it may flatten your caravan battery while you travel. But your beer might be nice and cool 😎

Thanks Guy's

I've changed the whole cable length, apart from the last couple of foot in the van and the new cable has 8x1.5mm cores and 5x2.5mm cores and i've made sure the right ones go to the correct pins. I will check the car socket to ensure that everything is good there.